1. Field of the Invention
This invention relates to recovery of petroleum from subterranean petroleum-containing formations. More particularly, this invention relates to a method for recovering petroleum from such formations by the injection of flood water containing a surfactant. A specific embodiment involves the injection of a viscous aqueous preflush solution of controlled salinity and/or hardness water having dissolved therein a hydrophilic polymeric material to efficiently displace the formation water.
2. Prior Art
Many subterranean, petroleum-containing formations contain natural energy in the form of active bottom water drive, solution gas drive, or gas cap drive, in sufficient quantity to drive the petroleum to the production well from which it can be transported to the surface. This phase of oil recovery, known as primary recovery, recovers only a portion of the petroleum originally in place. When the natural energy source has been depleted, or in those formations where insufficient natural energy was originally present to permit primary recovery, some form of supplemental treatment is required to recover additional oil from the formation. Water flooding is by far the most economical and widely practiced supplemental recovery procedure. Water flooding is accomplished by injecting water into the formation via one or more injection wells. The injected water displaces and moves the petroleum toward one or more production wells, where it is transported to the surface. Water flooding is also quite inefficient, and 50 percent or more of the original oil usually remains in the formation of the termination of conventional water flooding operations.
Numerous factors are responsible for the failure of water flooding to recover a higher percentage of the oil remaining in the formation after primary recovery. A low viscosity fluid displaces a higher viscosity fluid quite inefficiently, because the low viscosity displacing fluid channels through the high viscosity fluid. The displacement efficiency can be related mathematically to the mobility ratio of the displacing and displaced fluids. Various additives have been proposed in the prior art to alleviate this problem. Hydrophilic polymers which increase the viscosity of the displacing fluid, improve the mobility ratio and decrease the tendency for the displacing fluid to channel or finger into an inefficiently displace the higher viscosity petroleum. U.S. Pat. No. 3,039,529 (1962) discloses the use of polyacrylamide polymer to increase the viscosity of injected water to improve the mobility ratio and hence the oil displacement efficiency of an oil recovery process. U.S. Pat. No. 3,282,337 describes the use of polyethylene oxide as a thickener for injection water for the same purpose.
The immiscibility of water and petroleum, and the high surface tension existing between water and petroleum is a major cause of the inefficient displacement of oil by water. The use of a surfactant to lower this surface tension will improve the displacement efficiency. For example, U.S. Pat. No. 2,233,381 (1941) discloses the use of polyglycol ether as a surfactant in an oil recovery process. U.S. Pat. No. 3,032,713 (1967) discloses the use of a particular petroleum sulfonate as a surfactant for oil recovery. U.S. Pat. No. 3,468,377 describes the use of petroleum sulfonates having a specified molecular weight distribution as a surfactant for oil recovery.
The combined use of a surfactant solution to decrease the surface tension between the injected aqueous fluid and the petroleum contained in the formation, and a solution of a polymeric material to improve the mobility ratio and displacement efficiency provide a very efficient petroleum recovery process. For example, U.S. Pat. No. 3,477,511 (1969) describes the use of a surfactant solution followed by thickened water to displace the surfactant solution through the formation. Many other combinations of surfactants and water thickening polymers have been proposed, all sharing the common feature of specifying that the surfactant must precede the viscous fluid for optimum recovery efficiency.
Most surfactants proposed in the prior art for reducing the surface tension between the injected aqueous fluid and the formation petroleum, require a fluid environment whose salinity and/or hardness is within specific values in order to function with optimum effectiveness. For example, many of the petroleum sulfonates which would otherwise be the surfactant of choice in supplementary oil recovery operations be cause of their high surface activity and relatively low cost, require a salinity less than 2 percent by weight and a hardness less than 500 parts per million to function effectively, and their optimum performance is realized only at even lower salinity and hardness values. As an illustration of the recognition of this problem, a paper presented at the Society of Petroleum Engineers meeting at Tulsa, Oklahoma, in April, 1972, entitled "A Field Test of Surfactant Flooding" by S. A. Pursley, R. N. Healy, and E. I. Sandvik, describes a field pilot test employing a petroleum sulfonate surfactant in a biopolymer polymeric thickening material. The surfactant chosen required a lower salinity environment than was present in the formation to achieve the maximum reduction in surface tension. A low salinity or fresh water preflush was used in an attempt to reduce the salinity of the formation water in advance of the aqueous solution containing the surfactant. The authors of the paper concluded that the fresh water preflush was not a satisfactory method of reducing the formation water salinity to a level which would permit the surfactant to function at its maximum effectivenss, and expressed the opinion that the only satisfactory solution would be to utilize a surfactant system having an inherently high tolerance to salinity. The salinity of the formation water in the particular field test described in this paper was approximately 100,000 parts per million or 10 percent by weight. While this is a high salinity, certainly too high to permit the use of a high salinity sensitive surfactant such as petroleum sulfonate, this is by no means an abnormally high salt content for petroleum reservoirs. Reservoirs are known in which formation waters have salt concentrations of 250,000 parts per million or 25 percent by weight. Many such reservoirs also contain other interfering ions such as divalent ions including calcium and magnesium, which interfere with the proper function of many surfactants such as petroleum sulfonate.
U.S. Pat. No. 3,482,631, Jones, teaches the use of an aqueous preflush solution containing a hydrophilic polymer as a viscosifier in advance of a micellar dispersion or an emulsion type of displacing fluid in order to displace interfering ions including monovalent (sodium or potassium) or divalent ions (calcium or magnesium). It is well known that in two phase systems such as emulsions or micellar dispersions, that surfactants accumulate at the interface between the continuous and discontinuous phases, and so in emulsions having 30-60 percent by volume hydrocarbon as the discontinuous phase, the surfactant is concentrated in the interfacial zone and so it would be expected that surfactants would exhibit much less sensitivity to interfering ions in the instance of use emulsion type displacement fluids than when present in single aqueous phase displacement fluids. The micellar dispersions are special types of emulsions wherein the discontinuous (usually non-aqueous) phase is dispersed to a higher degree than more conventional emulsions. Although they are frequently described as "true solutions" because they appear clear, in fact the discrete discontinuous non-aqueous phase is still present. The "solution-like" appearance of miclellar dispersions results from much smaller "particle" sizes of the dispersed phase. Thus in comparing emulsions and micellar dispersions having equivalent total non-aqueous phase content, the micellar dispersion would have a larger number of discrete zones of the non-aqueous phase and hence the interfacial zone would be larger in area than that of the emulsion. Thus the surfactant present in a micellar dispersion would be even more prone to accumulate at the interfacial zone than it would in a conventional emulsion, and so little sensitivity to water hardness of salinity would be expected. The cost of operating a tertiary recovery project using an emulsion or micellar dispersion as the displacing fluid is usually prohibitively large, however, because of the large concentration of hydrocarbons and so there is a substantial commercial need for a method for using essentially single, aqueous phase surfactant solutions in oil recovery without incurring problems from salinity or hardness of formation water.
The salinity sensitivity of the most desirable surfactants for use in oil recovery has a substantial impact on the economics of a proposed supplemental recovery operation employing a surfactant. While it has been generally recognized in the industry for many years that surfactants capable of reducing the interfacial tension between the injected fluid and the formation petroleum would improve the oil recovery efficiency of a supplemental oil recovery program, it has never been demonstrated that the additional oil which can be recovered under field conditions is sufficient to justify the cost of the surfactant. This is especially true because of the enormous quantity of surfactant which must be employed in a field, in order to have a significant effect on the displacement efficiency. If high formation water salinity results in a shaft in surfactant choice to a higher cost material or if a greater concentration of surfactant must be used, the cost of a surfactant flood will be increased substantially. It is known, however, that many millions of barrels of oil remain unrecovered in a petroleum reservoir at the conclusion of conventional water flooding operations, and with the current shortage of readily recoverable crude oil, it is becoming a matter of paramount national importance to devise a reasonably economical method of recovering this oil.